Refactoring and a little bug fix

* Make `ac-dcd-call-process` independent from `ac-dcd-parse-output`.
* Use `ac-dcd-call-process` if possible. (`ac-dcd-get-ddoc` doesn't work with `ac-dcd-call-process`.  It's under examination.)
* little bug fix: call `(goto-char (point-min))` before showing document.